#include <iostream>
#include <fstream>


using namespace std;

int n;
struct p
{
	double x,y;
};
p cows[100000];
double ans = 0;

#ifdef home
ifstream fin("input.txt");
ofstream fout("output.txt");
#else 
ifstream fin("aria.in");
ofstream fout("aria.out");
#endif

int main()
{
	fin>>n;
        for (int i=0; i<n; i++)
	{
		fin>>cows[i].x>>cows[i].y;
	}
	for (int i=0; i<n; i++)
	{
		double x=0;
		x = 
		(0.0+cows[i%n].x*cows[(i+1)%n].y-cows[i%n].y*cows[(i+1)%n].x)/2.0;
		ans +=x;
	}

	fout<<ans<<'\n';

	return 0;
}